Optical communications systems are presently of commercial importance because of their large information carrying capacity. Optical communications systems as presently contemplated have a light source and a photodetector which are optically coupled to each other by means of a glass transmission line which is commonly referred to as an optical fiber. Systems presently in use carry information at rates in excess of 100 Mbit/sec and it is contemplated that future systems will carry information at rates geater than 1 Gbit/sec.
For highest transmission rates and longest distances between light source and photodetector, the light source presently preferred by those skilled in the art is a semiconductor laser diode. These diodes are relatively compact and can emit radiation with a relatively narrow spectral width in the wavelength regions presently of greatest interest. Diodes can now be fabricated having both single transverse and single longitudinal mode output. Such diodes are commonly referred to as single frequency lasers and are desirable in many applications because they, for example, maximize light coupled into the fiber and minimize deleterious aspects of the fiber dispersion characteristics. These characteristics may broaden the light pulse and thus limit the attainable bit rate and distance between source and detector. If either the bit rate of the distance between source and photodetector become too great, adjacent light pulses will overlap because of fiber dispersion and information is lost. Although a variety of modulation techniques has been proposed, present systems use intensity modulation (IM) of the laser output to convey information. That is, information is conveyed by variations in the intensity of the light output from the laser.
However, other modulation techniques offer specific advantages over intensity modulation. For example, higher frequency modulation is possible with frequency modulation (FM) than with IM for at least two reasons. First, the combination of the inherent FM or IM response with the RC parasitics results in a more efficient high frequency response with FM than with IM. Second, the rolloff above resonance is slower for FM than for IM.
Moreover, direct intensity modulation of a semiconductor laser becomes increasingly difficult as the bit rate increases, i.e., as the frequency increases. Direct intensity modulation means that the intensity of the light output is varied by varying the current through the laser. This type of modulation has at least three problems which become more significant at high bit rates. First, current modulation causes frequency modulation of the semiconductor laser diode which broadens the spectral width of the emitted radiation. This effect is commonly termed chirping and may be as large as, for example, 5 Angstroms. Chirping is often undesirable because of the dispersive properties of the fiber. Second, to intensity modulate the laser, a large amount of current, typically 20 mA or more, must be rapidly switched and this switching becomes more difficult as the frequency increases. Third, many single frequency lasers, such as the cleaved coupled cavity and external cavity lasers, cannot be fully intensity modulated with ease because of laser mode hopping, i.e., the laser output shifts from one longitudinal mode to another. This is commonly termed the "extinction penalty."
Because of these reasons, alternatives to direct intensity modulation have been considered. One alternative commonly contemplated today is the use of an external modulator which might be, for example, an integrated optic modulator. The laser emits radiation continuously and the desired intensity modulation is supplied by singnals to the modulator which vary light absorption within the modulator. However, high voltages, typically greater than 10 volts, are often required for efficient operation of the external modulators presently contemplated at high frequencies. The voltages required generally increase as the frequency increases. Additionally, there is also the problem of obtaining simple, efficient, high speed modulators. Finally, there is the additional problem of signal loss resulting from the coupling between the laser and modulator as well as between the modulator and the fiber.
Yet another approach uses coherent optical techniques which require locking two oscillators together, i.e., the two oscillators must be at the same frequency. While high sensitivity is obtained, locking the oscillators together may be difficult as they may be more than 100 km apart.